Public cellular networks (public land mobile networks) are commonly employed to provide voice and data communications to a plurality of subscribers. For example, analog cellular radiotelephone systems, such as designated AMPS, ETACS, NMT-450, and NMT-900, have been deployed successfully throughout the world. More recently, digital cellular radiotelephone systems such as designated IS-54B and IS-136 in North America and the pan-European GSM system have been introduced. Several systems for providing wireless voice and data communications are described, for example, in the book titled Cellular Radio Systems by Balston, et al., published by Artech House, Norwood, Mass., 1993. In addition, satellite based radio communication systems are also being utilized to provide wireless communications in various regions such as the Asian Cellular Satellite System (ACeS).
Typically, modern cellular systems use communication protocols which receive messages as frames which either singly or in combination contain a message. An example of one such communication protocol used in modern wireless communication systems is the IS-136 standard. IS-136 is the TDMA (Time Division Multiple Access) Digital Analog Mobile Phone System Cellular (D-AMPS) specification. One feature introduced with IS-136 is a layered protocol for the digital control channel. These three layers are illustrated in FIG. 1 and include the Physical Layer (L1), the Data Link Layer (L2) and the Call Processing Layer (L3).
The Physical Layer (L1) provides access to the wireless communication media which is a 4,860 bits/second .pi./4 Differential Quadrature Phase Shift Keying (DQPSK) channel. The Physical Layer includes a physical channel that is divided into 40 ms frames, each of which is sub-divided into 6 slots. Data is convolutionally encoded which allows for 125 bits of Layer 2 data per slot of the physical channel if the CRC, L3 Data and L2 header are included.
The Data Link Layer (L2) monitors and controls reverse accesses, decodes L2 messages and buffers the resultant L3 data, filters messages not destined for the mobile station and controls the mobile low power duration. L2 resolves messages into either an Fast Broadcast Control Channel (FBCCH), an Extended Broadcast Control Channel (EBCCH), or a SMS,PCH and ARCH Channel (SPACH) message. FBCCH and EBCCH messages are broadcast messages which are intended for all mobile stations. SPACH messages are addressed to a specific mobile station, for example, incoming calls are SPACH messages. SPACH messages are further divided into Paging Channel (PCH) messages which are used for initially notifying a mobile station of an event, Short Message Service Channel (SMSCH) messages which are used for transmission of short messages, or Access Response Channel (ARCH) messages which are used for conveying responses to the mobile station after the mobile station has transmitted a message.
The Call Processing Layer (L3) is a state machine which defines the state of the mobile station. The mobile station waits for incoming notices by "camping" on a control channel. The mobile station leaves the camping state to handle an incoming PCH message, to handle an origination attempt, to register, or to send a short text message, etc.
As is briefly described above, an IS-136 system receives frames which are decoded by L2 which also buffers the L3 data of a message. Thus, L2 assembles messages from the frames and passes the messages to L3. L3 then carries out the operations specified by the messages received from L2. One difficulty with such a system may arise where fewer than all of the frames of a message are received by L2 such that a complete message cannot be assembled and forwarded to L3 (i.e., some L3 data is lost). Currently, systems known to the inventors discard these incomplete messages either at L2 or L3. Thus, the loss of a message, which may span multiple frames, may result from the loss of a single frame. In transmission environments where the loss of frames is common, this loss of messages typically reduces the efficiency of communication.
In light of the above discussion, a need exists for improvements in the processing of messages from frames received in a wireless communication system so as to reduce the likelihood that messages will be lost.